1. Technical Field
The present invention relates to phosphors and, more particularly, to phosphors for use in a lighting apparatus.
2. Description of Related Art
Semiconductor lighting apparatuses include light-emitting diodes (LEDs) and laser diodes. Semiconductor lighting apparatuses which provide ultraviolet or near ultraviolet light can be used in combination with different phosphors to make various kinds of light sources.
Of all the new products in the LED industry, white light-emitting diodes have the greatest potential in commoditization and popularization because they provide such advantages as having a small size, low heat generation, low energy consumption and long glowing persistence. Therefore, white light-emitting diodes can be applied to the illumination for daily use and hopefully replace fluorescent lamps and the conventional light sources used in the back lights of flat-panel displays. The so-call “white light” is in fact a combination of various color lights. A white light visible to human eyes must comprise a combination of at least two categories of lights of different colors, such as a combination of blue and yellow lights or a combination of green, blue and red lights.
Presently, most of the commercial white lighting apparatuses generate a white light by using a phosphor powder of Y3Al5O12:Ce (YAG:Ce), which emits a yellow light when excited and combined with a blue LED. This commercial yellow-emitting phosphor powder is prepared through a solid-state sintering reaction at a high temperature ranging from 1400° C. to 1600° C., and can be excited by a blue LED having an emission wavelength of 467 nm to produce a yellow light having an emission light of 550 nm, whose CIE chromaticity coordinate is (0.48, 0.50).
This yellow-emitting phosphor powder for use with a blue LED is synthesized under a strict condition, namely, through a solid-state sintering reaction at a relatively high temperature. Moreover, this yellow-emitting phosphor powder shows a poor color rendering property when used in a white-light lighting apparatus because the powder emits a light that lacks a blue light component.
As to the commercial blue-emitting phosphor powder, (Ba0.8Eu0.2)MgAl10O17 (abbreviated as BAM) of Kasei Optonix, a Japanese phosphor company, has the widest applications. In addition to application in semiconductor lighting apparatuses, BAM is indispensable in plasma display devices. However, BAM has such disadvantages as a short glowing persistence and being unstable under an ultraviolet light. Still worse, the maximal emission wavelength of BAM tends to shift towards a longer wavelength during the manufacturing process, so that the color performance of BAM is biased to a green light and thus results in a lower color purity.